1. Field of the Invention
The present invention relates generally to supersonic aircraft engine exhaust systems which require noise suppression and particularly relates to the provision of ejector chutes permanently mounted within the exhaust stream for mixing ambient air with the exhaust gasses.
2. Description of Prior Development
Aircraft engines designed for supersonic flight at speeds of Mach 2.0 to Mach 4.0 produce high noise levels during take-off. For commercial applications, this noise must be suppressed to meet governmental noise level limits.
One known method of noise suppression places ejector chutes in the hot exhaust gas stream to entrain ambient air and enhance mixing of the air and exhaust gas. This entrainment of air increases the total mass flow exiting the nozzle while decreasing the exit velocity of the exhaust gas. The decreased exit velocity results in lower noise levels while the increased mass flow maintains the required take-off thrust.
Previous supersonic exhaust nozzle designs employing ejector chutes for air entrainment and noise suppression have used movable chutes that were stowed out of the exhaust stream during modes of operation not requiring noise suppression such as during transonic acceleration, subsonic cruise and supersonic cruise. A problem associated with the use of such movable chutes is the limited space available for their stowage. Thus, their size and ambient air entrainment capabilities are limited.
A measure of the air entrainment capabilities of ejector chutes is defined as the blockage ratio. This is the total flow area at the downstream exit of the ejector chutes divided by the core flow throat area at take-off. An increase in the blockage ratio will tend to increase ambient air entrainment and decrease exhaust noise. For chutes stowed during non-noise suppression operation, maximum blockage ratios are approximately 2.25.
One way to increase the blockage ratio of the ejector chutes is to leave the chutes in the hot exhaust gas stream during both suppressed and non-suppressed operation. With this arrangement, a blockage ratio of approximately 3.0 can be achieved thus improving the ambient air entrainment and noise suppression.
Previous designs that have permanently maintained chutes in the hot exhaust gas stream have employed the aft ends of the chutes to vary the exhaust nozzle throat area and exhaust nozzle internal exit areas throughout take-off, acceleration, subsonic and supersonic modes of operation. Such designs have also used a fixed external nozzle exit area. This results in less than optimum performance during all modes of engine operation.
Accordingly, a need exists for an exhaust system for a high speed civil transport aircraft engine having permanently maintained ejector chutes which provide a high blockage ratio yet which also allow for good engine performance during the acceleration, subsonic and supersonic modes of operation.